1. Field of the Invention
The present invention relates to communications systems in general and more particularly to multi-loop communications systems.
2. Prior Art
In the past, several types of communications networks have been proposed or used for transmitting data between a plurality of data sources. The single ring or serial loop configuration is one of the prior art arrangements. Essentially, this type of communications system includes a communication channel formed into a loop. One or more data sources are connected to adapters and the adapters are connected to the communication channel or wire. The adapters are arranged serially about the periphery of the loop. The configuration is such that data flows from a data source through its associated source adapter and serially through one or more intervening adapters. When the data reaches the target adapter, it is extracted from the loop and is subsequently forwarded to the target data source. If there are no intervening adapters, the data flows directly from the initiating data source/adapter through the loop and into the target data source/adapter. A more detailed description of single loop configurations is given in U.S. Pat. No. 3,890,471, issued to Victor Hachenburg on June 17, 1975. The copending patent application, Ser. No. 326,291, filed on Dec. 1, 1981, by P. A. Janson et al, entitled "Method of Transmitting Information Between Stations Attached to a Unidirectional Transmission Ring," and Ser. No. 463,470, filed on Feb. 3, 1983, by Tucker et al, entitled "Protocol for Determining Physical Order of Active Stations on a Token Ring," describe techniques for managing a single loop communication network. Both pending applications are assigned to the assignee of the present invention.
Another prior art configuration is the multi-loop or multi-ring arrangement. The multi-loop arrangement is best suited for an installation where a large number of data sources are to be connected. It is believed that the multi-ring configuration is more efficient than the single loop for handling large number of data sources hereinafter called Data Terminal Equipment (DTE). The multi-loop arrangement consists of a plurality of independent single ring communication network interconnected to one another by one or more switching stations called bridges. Any data source or terminal on one ring may transfer messages to another terminal on the same or different rings. The switching of messages between rings is provided by the switching stations (bridges) which interconnect the rings.
U.S. Pat. No. Re.028,811 (Ser. No. 410,813, filed Oct. 29, 1973, issued on May 11, 1976 to John Robinson Pierce) is an example of the prior art multi-loop communiciation system. In the patent a plurality of independent rings are interconnected, by switching stations, to form a single network. Messages are conveyed between stations by standardized message blocks. Each message block includes one or more address fields located at the head or beginning of each block. The switching stations use the contents of the address field for switching the messages between rings. A difference criterion is used for switching the message. Essentially, the switching station detects a destination code which is carried in the address field. The detected destination code is then compared with the code for the loop on which the message block is currently circulating. If the codes are different, the message block is switched to another interconnecting loop. The process continues until a match is obtained. This technique does not ensure that the message will traverse the optimum (i.e., shortest) path between source and destination.
U.S. Pat. No. 3,742,144 (Ser. No. 201,744 filed Nov. 24, 1971) describes another technique for routing messages in a digital multi-loop communication network. A Hamming distance criterion is used as the basis for switching messages between the loops.
U.S. Pat. No. 4,081,612 (Ser. No. 705,251 filed July 14, 1976, foreign priority Switzerland July 31, 1974) describes a technique for building up a routing address between stations interconnected by bearer channels and switching nodes.
U.S. Pat. No. 4,049,906 (Ser. No. 679,074 filed Apr. 21, 1976, foreign priority Switzerland Apr. 25, 1975) describes an apparatus for forwarding messages through the switching stations of a digital network with plural switching stations.
Other prior art techniques for routing messages in multi-ring networks are described by D. J. Farber and J. J. Vittal in an article entitled, "Extendability Considerations in the Design of the Distributed Computer System (DCS)," Proceedings, National Telecommunications Conference, November, 1973.
Still other prior art patents including U.S. Pat. Nos. 3,890,471; 4,287,592 and 4,510,492 disclose devices and methods for exchanging messages between communications rings.
The drawback with the prior art is that there is no mechanism which limits the number of copies of a message that will be delivered to a target ring. Thus, multiple copies of the same message will be delivered to a target station. This consumes bandwidth and causes unnecessary congestion on the network.